Polycarbonates have been known for a number of years. U.S. Pat. No. 3,301,824 describes the preparation of carbonate homopolymers and random copolymers with cyclic lactones. While the patent generally discloses the polymers as having utility in the molding, coating, fiber and plasticizing fields, there is no appreciation whatsoever of biodegradable and/or bioresorbable devices composed in whole or in part of polycarbonate.
Non-bioresorbable synthetic permanent vascular grafts have been available and are made of either Dacron (polyethylene terephthalate) or microporous Teflon (polytetrafluoroethylene). Various prostheses such as grafts, and especially those of small diameters for use in coronary bypass procedures, must have certain properties. These properties include physical and mechanical compatibility with the vessel to which they are connected, suturability, compliancy, ability to withstand pressure and pressure fluctuations, and flexibility. Required properties also include biocompatibility, sterilizability, and low toxicity, allergenicity, and mutagenicity. Still other required properties include appropriate durability, both in terms of "shelf life" after fabrication and appropriate durability after implantation. Problems which arise from a mismatch of a native vessel and a prosthesis include dilatation which may result in aneurysm formation and anastomotic hyperplasia, kinking and the like. Vascular grafts having internal diameters of 8 mm or more and made of biodurable materials have so far been the only successful prostheses for providing a conduit for maintaining continuous blood flow while inflicting a minimal hematologic trauma. Vascular grafts made of Dacron in current clinical use are constructed of knitted or woven Dacron fibers with open pores in the fabric which have to be closed or diminished by preclotting before implantation. Such prostheses have been used as vascular replacements, but only for the relatively larger arteries.
Bioresorbable polymers have been used in the fabrication of devices for implantation in living tissue for several decades. Medical application of such polymers include absorbable sutures, haemostatic aids and, recently, intraosseous implants and slow-release drug delivery systems, to name but a few. Use of such polymers has been extended to tissue regeneration devices such as nerve channels, vascular grafts, sperm ducts, fallopian tube ducts and the like. To be effective, these devices must be made from materials that meet a wide range of biological, physical and chemical prerequisites. The material must be bioresorbable at least in part, nontoxic, noncarcinogenic, nonantigenic, and must demonstrate favorable mechanical properties such as flexibility, suturability in some cases, and amenability to custom fabrication.
Various polymers have been proposed for use in the fabrication of bioresorbable medical devices. Examples of absorbable materials used in nerve repair include collagen as disclosed by D. G. Kline and G. J. Hayes, "The Use of a Resorbable Wrapper for Peripheral Nerve Repair, Experimental Studies in Chimpanzees", J. Neurosurgery 21, 737 (1964). Artandi et al., U.S. Pat. No. 3,272,204 (1966) reports the use of collagen protheses that are reinforced with nonabsorbable fabrics. These articles are intended to be placed permanently in a human body. However, one of the disadvantages inherent with collagenous materials, whether utilized alone or in conjunction with biodurable materials, is their potential antigenicity.
Other biodegradable polymers of particular interest for medical implantation purposes are homopolymers and copolymers of glycolic acid and lactic acid. A nerve cuff in the form of a smooth, rigid tube has been fabricated from a copolymer of lactic and glycolic acids [The Hand; 10 (3) 259 (1978)]. European patent application No. 118-458-A discloses biodegradable materials used in organ protheses or artificial skin based on poly-L-lactic acid and/or poly-DL-lactic acid and polyester or polyether urethanes.
U.S. Pat. No. 4,481,353 discloses bioresorbable polyester polymers, and composites containing these polymers, that are also made up of alpha-hydroxy carboxylic acids, in conjunction with Krebs cycle dicarboxylic acids and aliphatic diols. These polyesters are useful in fabricating nerve guidance channels as well as other surgical articles such as sutures and ligatures.
U.S. Pat. Nos. 4,243,775 and 4,429,080 disclose the use of polycarbonate-containing polymers in certain medical applications, especially sutures, ligatures and haemostatic devices. However, this disclosure is clearly limited only to "AB" and "ABA" type block copolymers where only the "B" block contains poly(trimethylene carbonate) or a random copolymer of glycolide with trimethylene carbonate and the "A" block is necessarily limited to glycolide. In the copolymers of this patent, the dominant portion of the polymer is the glycolide component.
U.S. Pat. No. 4,157,437 discloses high molecular weight, fiber-forming crystalline copolymers of lactide and glycolide which are disclosed as useful in the preparation of absorbable surgical sutures. The copolymers of this patent contain from about 50 to 75 wt. % of recurring units derived from glycolide.